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New details emerge as a few Bolt EV packs continue to fail

A few Chevrolet Bolt EVs are continuing to have battery pack failures that can cause unexpected and relatively sudden loss of power while driving, according to General Motors and individual owner reports on social media sites.

Updated May 11, 2018

A new GM software update has been issued for essentially all 2017 and 2018 Bolt EVs related to the condition described in this article.

The problem is occurring primarily in early-built cars but is also occasionally happening in cars built many months after manufacturing began in late 2016, according to GM spokesman Christopher Bonelli.

“The underlying issue of the early production batteries is a cell defect resulting in low cell voltage”, Bonelli said. “This manifests itself by presenting an inaccurate reading of remaining range at lower states of charge that may cause drivers to lose propulsion sooner than they would expect”.

GM is actively using the OnStar computer on every Bolt EV to monitor and report early indications of cell failure. Bonelli said OnStar advisors who reach out to proactively warn specific Bolt owners of a problem with their battery packs may suggest they keep their battery state of charge above 30 percent. This may minimize the risk of unexpected propulsion power loss before they can bring their cars in for service.

“Of the nearly 30,000 Bolt EVs on the road, the vast majority of affected vehicles have early production battery packs” and “a large majority of customers have been contacted by OnStar prior to experiencing loss of propulsion”, he said. “In August [2017], we said that less than 1 percent of [10,000] Bolt EV owners had experienced loss of propulsion as a result of the battery voltage issue. Since then, this ongoing issue remains a small percentage of the Bolt EVs on the road”.

Bonelli declined to characterize the total number of cars that have developed the problem so far or to define the range of manufacturing dates that GM considers to be “early”.

Sean Graham, a member of the Chevy Bolt EV Owners Group on Facebook, recently experienced the problem in his car which was built in August, 2017. It had only been driven around 2,500 miles since it was delivered to him in October. He said he was not contacted proactively by OnStar but he actually knew he had a suspicious battery cell before the car first showed outward symptoms.

With a background in computers he had discovered just a week before the failure how he could monitor the voltage levels of every individual cell group in his car’s battery pack using the car’s diagnostic port. According to Graham, it’s possible that his efforts to monitor the cell group voltages may have inhibited OnStar’s normal automatic monitoring.

A view of the 2017-2018 Chevrolet Bolt EV battery pack with the top cover removed.

Almost every car sold has what is known as an OBD or On-Board Diagnostic connector which can usually be found underneath the steering wheel area inside the vehicle. On the Bolt, the connector is near the driver’s door. These sockets were originally required for emission control compliance testing but are now widely used by auto service technicians when diagnosing problems.

The OBD connector taps into the car’s internal computer network which is known as a Controller Area Network (CAN) bus. In modern vehicles there are often multiple CAN buses that are interconnected in various ways for communications performance or security reasons.

The connector allows someone with the correct diagnostic hardware to probe various internal vehicle data such as coolant temperature, vehicle speed, engine or motor rpm, and battery voltages. OBD readers can either be dedicated devices or they can use Bluetooth or WiFi networking to transmit CAN bus messages to and from general purpose computers like laptops, tablets, or smartphones so that special software applications can process and display the data.

Graham attached a Bluetooth-enabled OBD II reader to his Bolt EV and used it with an Android app known as Torque Pro. The app uses OBD identifiers called PIDs to query specific values from components within the car. A similar app named EngineLink is available for Apple iOS devices and other similar apps are available as well.

The Bolt EV battery contains 96 cell groups. A cell group is made up of 3 adjacent battery cells that are electrically connected together in parallel as if to make one larger cell. The cell groups are then electrically connected in series which has the effect of adding up each of their individual voltages into a total battery pack voltage that ranges from roughly 300V to 400V depending upon the state of charge of the overall battery.

Some PIDs were already well-known for the Bolt EV but the ones for reading individual cell group voltages were not among them. Graham used another app that scans a vehicle’s internal network looking for active PIDs. Using that tool, combined with some technical clues that he found on the Internet, he was able to scan and reverse engineer the PIDs for reading the individual cell groups. He has made the PIDs and related information that he gathered available online.

The Torque Pro display around the time of the first power reduction in Sean Graham’s Bolt EV with the battery at 17.91 percent of its full capacity. Cell group 88 (circled in red) shows a voltage of 2.799V while other cell groups being monitored are about 3.44V.

With Torque Pro configured with his newly discovered PIDs, Graham was now able to see that one of the cell groups in his pack had lower voltage. When not under load and at higher states of charge cell group 88 was about 0.2V lower than the other cell groups. At lower states of charge and under heavy load the weak group dropped up to 0.3 or 0.4V.

Normally a combination of hardware and software in the battery management system attempts to “balance” each cell group to keep them at approximately the same voltage but it is a slow process and is meant to handle cell groups that are losing capacity at only slightly different rates.

Graham typically kept his car charged at over 50 percent. When his car first experienced the power loss symptoms he was driving on the freeway with the battery under 25 percent full. In his case, the power reduction came in stages over several minutes so he was able to exit the freeway at reduced speed and find a charging station. After charging the car, he was able to drive under normal power until he could get it to his dealer to have the battery pack replaced under warranty.

Although GM has not provided a detailed description of the issues underlying the low cell voltage problem, it could be a symptom of a cell manufacturing defect. The faulty cell, over time, may become less and less able to hold energy compared to other cells in the pack.

That reduced energy capacity may cause its cell group’s voltage to drop faster than the other cell groups as the overall battery pack energy is depleted while driving. The weakened cell and cell group may “age” faster due to the added stress. Eventually, as the pack energy is used up while driving, the cell group containing the weak cell may run very low on energy while other cell groups still have substantial energy left.

When the type of lithium-ion battery cells used in the Bolt EV are near empty they normally decline down to around 3 volts. Shortly after that, their voltage drops downward at a much steeper rate as the last bit of energy continues to be drained.

The cells are usually not allowed to drop much below 2.75 volts. Cells which drop substantially below 2.5V can undergo irreversible chemical structure changes that damage the cathode and further reduce the cell’s ability to function.

The car must stop drawing power when an individual cell group’s voltage drops very low even though the rest of the cell groups still have remaining energy and higher voltage levels because there apparently is no mechanism for an individual cell group to be disconnected from the rest of the pack.

Graham’s experience was unusual in that he had driven his Bolt EV down to a near empty state just a few weeks before his car’s bad cell problem became apparent. He did this intentionally while monitoring data from the diagnostic port as part of his effort to understand the intricacies of the car. At that time, the battery pack appeared to behave as expected. It wasn’t until later when he discovered how to read out the individual cell group voltages that he discovered he had a weak cell group.

Electric cars are designed to prevent significant damage to the battery cells if driven all the way to empty although doing that repeatedly may add stress on the cells that could lead to faster battery range loss eventually over a period of years. It should not have caused the pack failure that Graham experienced a few weeks later.

Some people have noticed that the Bolt EV battery pack has evolved through multiple GM part number changes. Today, the battery pack can be ordered under two different replacement part numbers.

The battery coolant connectors on the May, 2017 version of the Bolt EV battery pack. The snapshot comes from a battery disassembly video by Weber State University.

GM’s Bonelli said these part number changes are “an effort to be as transparent as possible within our engineering and supply chain operations. The part number change was not related to performance or [to] this cell quality issue”.

For example, around May of 2017 one new part number was issued due to a design change in the connectors for the water-based liquid that actively cools (or can sometimes warm) the Bolt battery cells. One of the battery coolant connectors was changed to more clearly distinguish it from the other connector in order to prevent hose connection mistakes during vehicle assembly or repair.

Not every battery pack failure is caused by the low cell voltage issue. A few have been associated with other components such as, ironically, the computer modules dedicated to monitoring the cell groups.